Industrial electrical power generators are large generators that often include water-based cooling systems for the stators. These stator cooling water (SCW) systems can provide a flow of cooling water through fine tubes to cool the windings in the stator, e.g., through hollow strands of the stator winding coil or the like. The cooling fluid removes heat from the stator windings that is generated by the large electrical currents flowing through the windings. The SCW system typically includes a network of conduits that connect the tubes in the stators to a reservoir, pump system, heat exchanger, and filter/strainer arrangements. It is important that these fine tubes, as well as any filter or strainer arrangements, remain open and clear of obstructions to ensure a continuous and sufficient flow of cooling water to all parts of the stator.
During operation of industrial generators, deposition of copper oxides within the fine tubes and conduits of the SCW system can occur. These precipitates can form by dissolution of small amounts of copper (Cu) from the conductive stator windings into the cooling water, and precipitation of copper oxides onto internal surfaces of the SCW system. Such precipitation can occur, e.g., in the portion of the system leading into the stator tubes, where the cooling water is at a relatively low temperature and dissolved copper ions may be supersaturated in the water. In high-oxygen systems (e.g., systems having a relatively high dissolved oxygen concentration in the cooling water of greater than about 2000 μg/l (ppb), such as with air-saturated water), the precipitates formed are predominantly cupric oxide (CuO). In low-oxygen systems (e.g., where the dissolved oxygen content is less than about 50 μg/l (ppb)), the precipitates are mostly cuprous oxide (Cu2O). These deposits can accumulate near the openings of the fine tubes in the stators, thereby reducing flow of cooling water therethrough and possibly leading to undesirable or dangerous overheating of the stator.
Cleaning or removal of copper oxide deposits in SCW systems can be important for maintaining proper operating conditions of the generator and avoiding overheating of the stator, by maintaining sufficient flow of the water coolant through the system. Further, shutdown and restarting of industrial generators can be a costly and disruptive process. Accordingly, it is desirable to remove copper oxide deposits in SCW systems without shutting down the generator, e.g., to provide a system and method for copper oxide removal that is compatible with operating conditions of the generator and can be achieved while the generator is operating normally.
Certain techniques for cleaning copper oxides from generator cooling systems have been described. For example, U.S. Pat. No. 6,596,175 describes a method for dissolving cupric oxide (CuO) by introducing CO2 into the cooling water to increase the CuO solubility, thereby dissolving some of the precipitated CuO. U.S. Pat. No. 6,554,006 describes a similar method for dissolving CuO precipitates in generator cooling systems by introducing agents such as mineral acid solutions or oxidants such as EDTA into the cooling water to dissolve the precipitated CuO. Such cleaning methods have been developed primarily to dissolve cupric oxide, and are not generally effective for cleaning or removal of cuprous oxide (Cu2O) precipitates.
Further, it is important to maintain a low ionic concentration in the coolant water of SCW systems during operation of a generator. Unwanted electrical flashover of stator currents to ground can occur, for example, if the high fields in the stator windings find a conductive path through the coolant water. For example, a typical upper conductivity limit for coolant water in generators is about 10 μS/cm. This limitation can reduce the efficacy of copper oxide removal using certain compounds that may increase ionic concentrations (and conductivity) of the coolant water.
Accordingly, it can be desirable to provide a system and method for removing cuprous oxide deposits from stator cooling systems of industrial generators and the like, where such removal can be achieved while the generator is operating.